Set a URL for cacheinvalidation.
Set a URL for cacheinvalidation.

The current URL is 'n/a' which, on the chrome://credits page will
just take the user to the top of the page, which is somewhat
confusing. Since this is now in-source, set the URL to the
README.chromium file itself, since that now contains the most useful
information.

BUG=chromium:476190

Review URL: https://codereview.chromium.org/1390403004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#353357}

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